The present invention relates to an auto-tracking system for a helical scan type magnetic recording and recording apparatus. More particularly, it relates to an auto-tracking systen for a magnetic recording and reproducing apparatus which affords the optimum reproduced output in spite of the curve of a recording track and the curve of the scanning locus of a reproducing head.
In a helical scan type magnetic recording and reproducing apparatus for domestic use (hereinbelow, termed `VTR`) tracking control has heretofore been performed in order for a video head to precisely scan a recording track on a magnetic tape. As tracking control means therefor, a system is known wherein the rotational speed of a capstan is controlled so that a control signal reproduced from the magnetic tape and the phase of rotation of the video head may become predetermined values.
Various dispersion factors, however, make it necessary to vary the optimum value of the phase between the tape and the head so as to attain compatibility. For this reason, an auto-tracking system which automatically adjusts the optimum value is required.
Such an auto-tracking system has hitherto been proposed in various forms. An example is an auto-tracking system wherein, as disclosed in the official gazette of Japanese Patent Application Laid-open No. 54-41114, a tracking control signal is detected using the envelope level of a reproduced signal, and the traveling phase of a magnetic tape is controlled with the detected signal so as to maximize the envelope level.
The auto-tracking system provides features that the reproduced signal envelope level is periodically sampled, that the difference between the current sampled value and the last sampled value is detected and that the traveling phase of the magnetic tape is changed according to the detected difference so as to shift the video head in a predetermined amount in the widthwise direction of the track. When the reproduced signal envelope level increases owing to the successive changes of the tracking phase, this phase is further changed in the same direction, and when the envelope level turns to decrease, the phase is changed in the opposite direction, whereby the maximum envelope output point is automatically searched for.
In such a prior-art auto-tracking system, the curve of the track and the curve of the scanning locus of the video head are not taken into consideration. Accordingly, in the mode of reproduction in the case where the curve of the track of a recorded video tape differs from the curve of the scanning locus of a video head in a VTR for reproducing the tape (in other words, where recording and reproduction are performed with apparatuses different from each other), the signal-to-noise ratio degrades especially in a part of a screen (noise bars appear in some cases). That is, when the track curve 16 in the recording apparatus and the scanning locus curve 17 of the video head 4 in the reproducing apparatus are in the opposite directions as illustrated in FIGS. 8 and 9, noise develops in the lower part (or upper part) of a television screen because the prior-art system performs the control with the sampled value which is the mean value of reproduced signal envelope levels over a whole predetermined period or which is a reproduced signal envelope level preferentially extracted in correspondence with the central part of the television screen.